Composite video signals, commonly used in video broadcasts or transmissions, contain image data and synchronization information. The image data typically includes a brightness signal (luminance, luma or Y) and a color signal (chrominance, chroma or C), where the color signal is modulated into a color sub-carrier and added to the brightness signal prior to transmission. The synchronization information includes horizontal sync pulses to define horizontal line-to-line transitions of the image data, and vertical sync pulses to identify field-to-field transitions. Accurate detection and processing of these horizontal and vertical sync pulses allows systems to properly display the image data.
FIG. 1 shows a typical format of composite video signal 10. Referring to FIG. 1, the composite video signal 10 includes a vertical blanking period 14 within fields 12 and 16. The vertical blanking period 14 includes vertical sync pulses 30 to identify a transition between the fields 12 and 16. When the composite video signal 10 is interlaced, the vertical sync pulses 30 may also be used to identify the polarity of field 16, e.g., as either even or odd. Each field 12 and 16 contains horizontal lines of image data 24 separated by corresponding horizontal blanking periods 22. Each horizontal blanking period 22 includes both a horizontal sync pulse 23 to identify a horizontal line transition and a color burst 25 for use in demodulating the color sub-carrier in the horizontal line of image data 24.
Typically, display systems generate synchronization signals from the horizontal and vertical sync pulses 23 and 30 for use in processing and display of the image data 24 within composite video signal 10. Since picture quality depends upon the accuracy of the synchronization signals relative to the composite video signal 10, any corruption of the composite video signal 10 can cause undesirable results when the image data 24 is displayed. For instance, when time-based errors or temporal shifts are present within the composite video signal 10, a rapid phase shift of the synchronization signals is needed to maintain a lock with the composite video signal 10. Under noisy signal conditions, however, detection of the sync pulses 23 and 30 becomes unreliable, and thus correction of synchronization signals may degrade lock with the composite video signal 10. Current display systems generate their synchronization signals optimizing one of the signal locking techniques, to the detriment of the other technique. For instance, display systems that rapidly compensate for time-based errors are susceptible to miscorrecting their synchronization signals during noisy conditions. Conversely, display systems that minimize correction to synchronization signals to reduce correction-errors during noisy signaling conditions, cannot rapidly compensate for time-based errors. Accordingly, a need remains for a system and method for improved horizontal and vertical sync pulse detection and processing.